Positive displacement flushable flow meter

Abstract

A positive displacement flushable flow meter has sensing elements that are rotated by and at a rate in accordance with a flow of fluid through the flow meter. The flow meter is particularly adapted for measuring the volume flow rate of coating material supplied to spray coating apparatus, and to permit the flow meter to be quickly flushed by means of alternate bursts of relatively high velocity air and solvent, without overdriving the sensing elements and damaging the flow meter, a bypass valve is provided. During a spray coating operation, the bypass valve is closed and all of the coating material flows past and rotates the sensing elements. During a flushing operation, the bypass valve is opened to divert some of the relatively high velocity flow of air and solvent around the sensing elements, while allowing a controlled and limited amount of air and solvent to move past and rotate the elements to clean them without causing them to be overdriven.

What is claimed is: 
     
       1. A positive displacement flow meter for measuring the volume flow of liquid coating material delivered to spray coating apparatus and for being flushed clean of coating material by fluidic flushing media, said flow meter comprising a housing having inlet passage means and outlet passage means; means for supplying either liquid coating material at a first volume flow rate and under pressure or fluidic flushing media at a second and greater volume flow rate and under pressure to said inlet passage means; movable liquid flow sensing means intermediate said inlet and outlet passage means for flow therethrough of coating material and flushing media from said inlet to said outlet passage means, said sensing means being moved in response to and at a rate in accordance with the volume flow rate of coating material or flushing media therethrough and generating an indication of the volumetric flow of coating material flowing therethrough; bypass valve means for selectively shunting a portion of the fluid in said inlet passage means around said flow sensing means to said outlet passage means; and means for operating said bypass valve means so that no fluid is shunted around said flow sensing means when coating material is supplied to said inlet passage means so that a portion of the fluid is shunted around said flow sensing means when flushing media is supplied to said inlet passage means. 
     
     
       2. A positive displacement flow meter as in claim 1, wherein said bypass valve means, when selectively shunting a portion of the fluid, establishes a shunt path between said inlet and outlet passage means so that only a portion of the flushing media supplied to said inlet passage means flows through said flow sensing means to said outlet passage means and the remainder of the flushing media flows through said shunt path. 
     
     
       3. A positive displacement flow meter as in claim 2, wherein said bypass valve means comprises a first channel in communication with said inlet passage means, a second channel in communication with said outlet passage means, and a valve member selectively movable by said operating means between a first position establishing a path and a second position interrupting said path between said first and second channels. 
     
     
       4. A positive displacement flow meter as in claim 1, wherein said movable flow sensing means comprises at least one flow sensing element that is rotated by fluid flowing through said flow sensing means and at a rate in accordance with the volumetric flow rate of the fluid, and said bypass valve means, in shunting a portion of the flushing media around said flow sensing means, permits only a limited amount of the flushing media to pass through said flow sensing means to clean said flow sensing means without causing said at least one sensing element to be rotated at an excessive rate. 
     
     
       5. A positive displacement flow meter as in claim 4, wherein said at least one sensing element comprises a pair of gears in meshing engagement and having tooth spaces in which fluid is carried from said inlet to said outlet passage means, and the rate of rotation of said gears, when no fluid is shunted around said flow sensing means, is in accordance with the volume flow rate of coating material through said flow sensing means, and including means for detecting the revolutions through which said gears are rotated to detect the volume flow of coating material through said flow sensing means. 
     
     
       6. A positive displacement flow meter as in claim 5, wherein said gears are rotated in response to a differential in the pressure of fluid between said inlet and outlet passage means, and said bypass valve means maintains a pressure differential of fluid when operated to shunt a portion of the fluid in said inlet passage means around said flow sensing means. 
     
     
       7. A method of operating a positive displacement flow meter having at least one sensing element that is rotated in response to and at a rate in accordance with the volume flow rate of fluid through the flow meter, wherein the flow meter measures the volume flow of liquid coating material supplied under pressure to a spray coating apparatus, if flushed clean of coating material by fluidic flushing media, and the at least one sensing element has a maximum rate of rotation beyond which damage to the flow meter can occur, said method comprising the steps of supplying liquid coating material under pressure and at a first flow rate through the flow meter to the spray coating apparatus; detecting the revolutions through which the at least one sensing element is turned by the coating material to measure the volume flow of the coating material through the flow meter to the spray coating apparatus; upon completion of supplying coating material, delivering fluidic flushing media under pressure and at a second and greater flow rate through the flow meter to clean the same of coating material; and, during performance of said flushing media delivering step, shunting a portion of the flushing media around the at least one sensing element so that the at least one sensing element is exposed to only some of the flushing media which is sufficient to clean the at least one sensing element of coating material but is less than sufficient to rotate the at least one sensing element faster than its maximum rate of rotation. 
     
     
       8. A method as in claim 7, wherein said shunting step comprises establishing a bypass path around the at least one sensing element. 
     
     
       9. A method as in claim 7, wherein the at least one sensing element is rotated in response to a differential in the pressure of fluid thereacross, and said shunting step maintains a differential in the pressure of flushing media across the at least one sensing element. 
     
     
       10. A positive displacement flow meter for measuring the volume flow of liquid coating material delivered to spray coating apparatus and for being flushed clean of coating material by fluidic flushing media, said flow meter comprising a housing having an inlet and an outlet; means for supplying either liquid coating material under pressure or fluidic flushing media under pressure to said inlet; movable fluid flow sensing means in said housing intermediate said inlet and outlet for flow therethrough of coating material and flushing media from said inlet to said outlet, said sensing means being moved in response to and by an amount in accordance with the volume flow of coating material or flushing media therethrough and generating an indication of the volumetric flow of coating material flowing therethrough; bypass valve means for selectively shunting a portion of the fluid from said inlet around said flow sensing means to said outlet; and means for operating said bypass valve means so that no fluid is shunted around said flow sensing means when coating material is supplied to said inlet and so that a portion of the fluid is shunted around said flow sensing means when flushing media is supplied to said inlet. 
     
     
       11. A method of supplying a measured volume of liquid coating material to spray coating apparatus, comprising the steps of supplying liquid coating material to the spray coating apparatus through a path including a movable fluid flow sensor that is moved in response to and by an amount in accordance with the volume flow of coating material therethrough; measuring the volume flow of coating material through the fluid flow sensor in accordance with the amount of movement thereof; upon the measured volume flow of coating material reaching a selected value, interrupting said supplying step and delivering fluid flushing media through the path and the fluid flow sensor to clean the same; and, during performance of said delivering step, shunting a portion of the flushing media in the path around the fluid flow sensor.